Photolysis method for producing NF4 PF6

ABSTRACT

A method of producing NF 4  PF 6  by ultraviolet photolysis at -196° C of a mixture of NF 3 , F 2  and PF 5 .

The invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the United States Navy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for producing materials and is particularly directed to an improved method for producing NF₄ PF₆.

2. Description of the Prior Art

Nf₄ ⁺ salts are the key ingredients for solid propellant NF₃ --F₂ gas generators, such as that taught by D. Pilipovich in U.S. Pat. No. 3,963,542 for chemical HF--DF lasers. Whereas NF₄ SbF₆ and NF₄ AsF₆ can be prepared with relative ease, using the methods of W. E. Tolberg et al, U.S. Pat. No. 3,708,570, and K. O. Christe et al, U.S. Pat. No. 3,503,719, these compounds suffer from the disadvantage of containing a relatively heavy anion, thus decreasing their performance in an NF₃ --F₂ gas generator. This disadvantage can be overcome by replacing the SbF₆ ⁻ or AsF₆ ⁻ anion by the lighter PF₆ ⁻ anion. The existence of this salt has previously been claimed by Tolberg et al. (U.S. Pat. No. 3,708,570), but their production process was so inefficient that they could not isolate an amount of material sufficient for its isolation, identification, and characterization.

BRIEF SUMMARY AND OBJECTS OF INVENTION

This problem of synthesizing NF₄ PF₆ is overcome by the present invention and an efficient method was found for producing NF₄ PF₆. The method of the present invention involves low-temperature uv-photolysis of a mixture of NF₃, F₂, and PF₅ according to: ##EQU1## This method provides NF₄ PF₆ of high purity.

Accordingly, it is an object of the present invention to provide an improved process for the production of NF₄ PF₆.

This and other objects and features of the present invention will be apparent from the following examples.

DETAILED DESCRIPTION OF THE INVENTION

In a typical experiment, NF₃ and PF₅ (27 mmol of each) were condensed into the cold (-196° C.) bottom of a pan-shaped quartz reactor. This reactor had a flat top consisting of a 3-in. diameter optical grade quartz window. The vessel had a side arm connected by a Teflon O ring to a Fischer-Porter Teflon valve to facilitate removal of solid reaction products. The depth of the reactor was about 1.5 in. and its volume was 135 ml. The uv source consisted of a 900-W air-cooled high-pressure mercury arc (General Electric Model B-H6) and was positioned 1.5 in. above the flat reactor suface. The bottom of the reactor was kept cold by immersion in liquid N₂. Dry, gaseous N₂ was used as a purge gas to prevent condensation of atmospheric moisture on the flat top of the reactor. As a heat shield a 0.25 in. thick quartz plate was positioned between the uv source and the top of the reactor. Fluorine (9 mmol) was added and the mixture was photolyzed at -196° C. for 1 h with a 900-W high-pressure Hg arc in the manner described above. After termintion of the photolysis, volatile material was pumped out of the reactor during its warm-up to room temperature. The non-volatile white solid product consisted of 10 mg of NF₄ PF₆.

The salt NF₄ PF₆ is a white, crystalline, hygroscopic solid, stable at room temperature, but rapidly decomposing at 245° C. Its characteristic x-ray diffraction powder pattern is listed in Table I. Its vibrational spectrum is listed in Table II and establishes the ionic nature of the salt, i.e. the presence of discrete NF₄ ⁺ cations and PF₆ ⁻ anions. This was further confirmed by ¹⁹ F nmr spectroscopy in HF solution which showed the triplet (J_(NF) = 230 H_(z) at φ = -217) characteristic for NF₄ ⁺ .

                  TABLE I                                                          ______________________________________                                         X-RAY POWDER DATA FOR NF.sub.4 PF.sub.6.sup.a                                  d obsd      d caldc    Int       h k l                                         ______________________________________                                         5.40        5.36       ms        1 1 0                                         4.55        4.53       s         1 0 1                                         3.91        3.89       vs        1 1 1                                         3.79        3.79       s         2 0 0                                         2.91        2.91       ms        2 1 1                                         2.65        2.65       m         1 0 2                                         2.40        2.40       vw        3 1 0                                         2.307       2.305      m         3 0 1                                         2.204       2.205      vw        3 1 1                                         2.171       2.171      mw        2 1 2                                         1.882       1.883      ms        3 0 2, 0 0 3                                  1.825       1.827      vw        3 1 2, 1 0 3                                  1.784       1.785      w         3 3 0                                         1.747       1.747      mw        4 1 1                                         1.685       1.685      w         3 2 2, 2 0 3                                  1.646       1.646      w         2 1 3                                         1.622       1.622      w         4 2 1                                         1.536       1.540      vw        4 1 2                                         1.485       1.486      vw        5 1 0                                         1.464       1.463      vw        5 0 1                                         1.437       1.437      w         5 1 1                                         1.408       1.407      vw        5 2 0                                         1.365       1.365      vw        5 2 1, 1 1 4                                  1.333       1.335      w         5 0 2                                         1.318       1.319      vw        4 4 0                                         1.302       1.304      vw        2 1 4                                         1.259       1.259      w         4 2 3                                         1.214       1.216      w         6 1 1                                         ______________________________________                                          .sup.a tetragonal, a = 7.577, C = 5.653A, Cu K.sub.α radiation Ni        filter                                                                   

                                      TABLE II                                     __________________________________________________________________________     VIBRATIONAL SPECTRUM OF NF.sub.4 PF.sub.6                                      Observed                                                                            Frequency (cm.sup.-1)                                                     Ir   Raman     Assignments for NF.sub.4.sup.+ (T.sub.d)                        __________________________________________________________________________     2380 vw        2ν.sub.3 (A.sub.1 + E + F.sub.2) = 2320                      2320 w                                                                         2005 w         ν.sub.1 + ν.sub.3 (F.sub.2) = 2008                        1765 w         ν.sub.3 + ν.sub.4 (A.sub.1 + E + F.sub.2) = 1769          1457 w         ν.sub.1 + ν.sub.4 (F.sub.2) = 1457                        1221 mw        2ν.sub.4 (A.sub.1 + E + F.sub.2) = 1218                      1166 vs                                                                              1168 (1.5)                                                                     1150 (0.8)                                                                              ν.sub.3 (F.sub.2)                                            1135 vw                                                                        1056 vw        ν.sub.2 + ν.sub.4 (F.sub.1 + F.sub.2) = 1049                    880 (0.2)                                                                               2ν.sub.2 (A.sub.1 + A.sub.2  + E) = 880                            849 (8.2)                                                                               ν.sub.1 (A.sub.1)                                            611 m                                                                                609 (7.4)                                                                               ν.sub.4 (F.sub.2)                                            608 m                                                                                441 (2.9)                                                                               ν.sub.2 (E)                                                                 Assignments for PF.sub.6.sup.- (O.sub.h)                        1590 w         ν.sub.1 + ν.sub.3 (F.sub.1u) = 1590                       1414 w         ν.sub.2 + ν.sub.3 (F.sub.1u + F.sub.2u) = 1413            1308 vw        ν.sub.1 + ν.sub.4 (F.sub.1u) = 1307                       842 vs                                                                               838 (1.5)                                                                               ν.sub.3 (F.sub.1u)                                           789 w                                                                          749 w 748 (10) ν.sub.1 (A.sub.1g)                                                 571 (0.8)                                                                               ν.sub.2 (E.sub.g)                                            559 s          ν.sub.4 (F.sub.1u)                                           474 vw                                                                               469 (1.2)                                                                               ν.sub.5 (F.sub.2g)                                           __________________________________________________________________________

Obviously, numerous variations and modifications may be made without departing from the present invention. Accordingly, it should be clearly understood that the form of the present invention described above is illustrative only and is not intended to limit the scope of the present invention. 

We claim:
 1. A process for the production of NF₄ PF₆ characterized by photolyzing a mixture of NF₃,F₂ and PF₅ having an approximate mol ratio of 3:1:3 at -196° C and removing unreacted volatile starting materials from the non-volatile NF₄ PF₆ product by pumping at ambient temperature. 